Global bio-diesel production by transesterification of fatty acid esters has increased by several folds in the last decade to partly substitute the use of fossil-derived diesel fuel. The byproduct, glycerol, in this process has emerged as an important building block for chemicals. For example, glycerol can be converted to several high value chemicals such as 1,2-propanediol, 1,3-propanediol, acrolein, and glyceric acid. Recently, lactic acid has emerged as another promising product from glycerol. Lactic acid (2-hydroxypropionic acid) is a platform chemical for several important commodity products such as biodegradable fibers, polylactic acid esters, and acrylic acid. Lactic acid is mainly produced (about 95% of world production) from sugars and sugar alcohols by the fermentation route which is slow and involves complex separation steps. See Datta, in Kirk-Othmer concise encyclopedia of chemical technology, Fifth ed., Vol. 1, Wiley-Interscience, NJ, 1324-1327 (2004).
Synthesis of lactic acid from glycerol has been described under oxygen flow conditions using an Au—Pt/TiO2 catalyst and 4:1 molar ratio of NaOH to glycerol. The prior art reports high selectivity (86%); however, it uses oxygen and relatively high molar ratios of NaOH to glycerol. See Shen et al., Efficient synthesis of lactic acid by aerobic oxidation of glycerol on Au—Pt/TiO2 catalysts, Chem. Eur. J., 16 7368-7371 (2010). The abstract of Liu et al., Method and special catalyst for production of lactic acid by using glycerol as raw material, Chinese Patent No. 101695657 A (2010) describes a process which uses oxygen, alkali and a catalyst. The formation of lactic acid from glycerol using a high pressure of hydrogen (40-60 bar), base and a heterogeneous catalyst has also been described in Maris et al., Glycerol hydrogenolysis on carbon-supported PtRu and AuRu bimetallic catalysts, J. Catal. 251 281-294 (2007) and Marincean et al., Glycerol hydrogenolysis to propylene glycol under heterogeneous conditions, Chemical Industries 115 (Catalysis of Organic Reactions), 427-436 (2007). The major limitations are the use of hydrogen at high pressure and the low selectivity to lactic acid (40-60%).
Hydrothermal conversion of glycerol to lactic acid, wherein aqueous glycerol is treated at high temperature (573 K) under alkaline conditions, has been investigated as an alternative to the fermentation route. See Kishida et al., Electrolysis of glycerol in subcritical water, Chem. Lett. 34 1560-1561 (2005); Enomoto et al., U.S. Pat. No. 7,829,740 and 20100047140, which are all incorporated by reference in their entirety. The hydrothermal conversion process is advantageous as it can directly use glycerol from the bio-diesel production process containing water and alkali as feedstock with no need for a separation step. However, this process operates at near-critical temperature for water (Tc=647 K), and the alkaline medium therefore causes severe corrosion of the reactors. See Shen et al., Effect of Alkaline Catalysts on Hydrothermal Conversion of Glycerin into Lactic Acid, Ind. Eng. Chem. Res. 48 8920-8925 (2009), which is incorporated by reference.
The reaction pathway for glycerol to lactic acid is shown below. Dehydrogenation of glycerol to glyceraldehyde is a key step in this reaction. It has been shown that high temperatures (greater than 550 K) are required in the hydrothermal process to convert glycerol to glyceraldehyde via glyceroxide ion as an intermediate. Rami´rez-Lo´pez et al., Synthesis of lactic acid by alkaline hydrothermal conversion of glycerol at high glycerol concentration, Ind. Eng. Chem. Res. 49 (14) 6270-6278 (2010). However, the decomposition of pyruvaldehyde and lactic acid are significant at that temperature, adversely affecting the selectivity to lactic acid.
